Ink jet recording ink, ink jet image-forming method and ink jet recording apparatus

ABSTRACT

The invention provides an ink jet recording ink containing a self-dispersion pigment having a lactone group and a carboxyl group, an organic carboxylic acid salt, water and a water-soluble substance having a coefficient of hydrophilicity-hydrophobicity of 0.37 or more as defined by the following equation (A), and having a surface tension of 34 mN/m or less. Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a 20% aqueous solution)−(Molar fraction of water in the 20% aqueous solution)]/ [1−(Molar fraction of water in the 20% aqueous solution)]  Equation (A)

TECHNICAL FIELD

The present invention relates to an ink jet recording ink, an ink jetimage-forming method and an ink jet recording apparatus, for formingimages on plain paper.

BACKGROUND ART

With the spread of an ink jet recording system, there is a demand forspeeding up of recording and improvement in the quality of recordedimages, such as images of characters and photographs, when recording isconducted on plain paper by ink jet recording.

For example, there is a demand for recording official documents,photographic images of digital cameras, and various kinds of informationput on internet websites on plain paper at high speed and on both sidesof the plain paper. There is also a demand for achieving such clearimage quality as in recorded images obtained with a laser beam printer.There is further a demand for obtaining a high image density whencharacter images are printed and providing sharp character images thatare inhibited from being defaced even when the character images aresmall.

In recorded images such as color photographs and tables, there is also ademand for inhibiting bleeding at a color boundary portion, which iscaused upon contact of a plurality of recording inks of different colorswith each other.

In order to meet such demands, there has been proposed an ink, whichcontains organic ultrafine particles having an average particle size of0.5 μm or less and having been internally and three-dimensionallycrosslinked and provides high-density images inhibited from bleeding onplain paper (see Japanese Patent Application Laid-Open No. 2004-195706).There has also been proposed an ink that contains apermeability-imparting agent wherein the content of thepermeability-imparting agent is made larger than the content in whichthe surface tension of the ink composition, which decreases according tothe increase in the amount of the agent, ceases to decrease (seeJapanese Patent Application Laid-Open No. 2003-301129).

DISCLOSURE OF THE INVENTION

The present invention relates to an ink jet recording ink, an ink jetimage-forming method and an ink jet recording apparatus, which aresuitable for use in forming a recorded image on plain paper. Problemssought for achievement are shown below.

-   1) An ink is fixed at high speed.-   2) A recorded image has a high density and is clear.-   3) A recorded image is inhibited from causing bleeding between inks.-   4) A recorded image is inhibited from causing strike through.-   5) Even when small characters are printed, the characters are sharp    and inhibited from being defaced.-   6) A recorded image has good water fastness and fixability.

According to the inks described in Japanese Patent Application Laid-OpenNos. 2004-195706 and 2003-301129, water fastness and inhibition ofbleeding between two (2) color inks are improved to some extent.However, with these inks, all the above six problems are not satisfied,and in particular, problems of making the density of recorded imagehigher, inhibiting the strike through of the recorded image andachieving good printing of small characters are left unsolved.

As described above, an ink jet image-forming method capable ofsufficiently satisfying all the above six problems, which have beendemands for achieving high-quality recording on plain paper in recentyears, at the same time is not found.

Accordingly, it is an object of the present invention to provide an inkjet recording ink, an ink jet image-forming method and an ink jetrecording apparatus, which sufficiently satisfy the above problems 1) to6) at the same time.

The above object can be achieved by the present invention describedbelow. More specifically, according to a first invention, the presentinvention provides an ink jet recording ink comprising a self-dispersionpigment having a lactone group and a carboxyl group, an organiccarboxylic acid salt, water and a water-soluble substance having acoefficient of hydrophilicity-hydrophobicity of 0.37 or more as definedby the following equation (A), and having a surface tension of 34 mN/mor less.Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)−(Molar fraction of water in the 20% aqueoussolution)]/[1−(Molar fraction of water in the 20% aqueoussolution)]  Equation (A)

According to a second invention, the present invention provides an inkjet image-forming method for forming an image by applying theabove-described ink in a fixed amount of from 0.5 pl or more to 6.0 plor less to plain paper, wherein the application of the ink is dividedinto plural times when an image having a portion where the ink isapplied in a duty of 80% or more and in an amount of 5.0 μl/cm² or lessin total is formed in a basic matrix for forming the image, and theamount of the ink applied at each of the times divided is 0.7 μl/cm² orless.

According to a third invention, the present invention provides an inkjet recording apparatus equipped with a recording head for forming animage by applying the above-described ink in a fixed amount of from 0.5pl or more to 6.0 pl or less to plain paper, wherein the apparatuscomprises a control mechanism, by which the application of the ink isdivided into plural times when an image having a portion where the inkis applied in a duty of 80% or more and in an amount of 5.0 μl/cm² orless in total is formed in a basic matrix for forming the image, and theamount of the ink applied at each of the times divided is controlled to0.7 μl/cm² or less

Effects of the Invention

According to the ink jet recording ink, ink jet image-forming method andink jet recording apparatus of the present invention, the clogging withan ink can be inhibited, and the fixing of the ink can be conducted athigh speed when the ink is applied to plain paper. It is also possibleto provide a clear and high-quality image having sufficient waterfastness and image density and inhibited from bleeding. Even when smallcharacters are printed, the resulting characters are sharp and can beinhibited from being defaced. It is further possible to form an imageinhibited from causing strike through and suitable for double sidedprinting. These are marked effects that are exhibited for the first timeby meeting all the above-described constitutional requirements of thepresent invention and unanticipatable from the prior art.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an ink jet recording apparatusaccording to an embodiment applicable to the present invention.

FIG. 2 illustrates recording heads applicable to an embodiment of thepresent invention.

FIG. 3 illustrates an example of a method for forming recording dots.

BEST MODES FOR CARRYING OUT THE INVENTION

The present inventors have carried out an investigation on an ink jetrecording ink, and ink jet recording method and apparatus that provideclear and high-quality images fixed at high speed to plain paper andhaving sufficient water fastness and image density, and are suitable fordouble side printing. As a result, it has been found that thecomposition of an ink that quickly causes solid-liquid separationbetween a pigment and an aqueous medium after impacted on the plainpaper, physical properties of the ink, the amount of the ink to beapplied, which is controlled on the side of a recording apparatus, andconditions for divided application of the ink are precisely controlled,whereby the above object can be achieved.

The present invention will hereinafter be described in more detail bypreferable embodiments.

<Ink>

(Coloring Material)

Coloring materials used in inks used in the present invention areself-dispersion pigments having a lactone group and a carboxyl group. Asa counter ion of the carboxyl group may be used a hydrogen atom, alkalimetal, ammonium or organic ammonium. Specific examples of the alkalimetal include Li, Na, K, Rb and Cs. Specific examples of the organicammonium include methylammonium, dimethylammonium, trimethylammonium,ethylammonium, diethylammonium, triethylammonium, monohydroxy-methyl(ethyl)amine, dihydroxymethyl(ethyl)amine andtrihydroxymethyl(ethyl)amine. Among the counter ions, ammonium isparticularly preferable.

When a general water-soluble dye is used, an image formed on a recordingmedium, to the surface of which cellulose fiber is exposed, such asplain paper is poor in water fastness. When the self-dispersion pigmentaccording to the present invention is used, however, the resulting imagecan have good water fastness. When compared with a pigment of aresin-dispersion system (a system using a resin as a dispersant incombination) that is another dispersion form of a water-dispersiblepigment, the self-dispersion pigment is considerably advantageous incolor developing because the fixing of a coloring matter componentcomposed of the pigment to a surface portion of paper is efficient by adegree that no dispersant is contained.

When the self-dispersion pigment having a lactone group and a carboxylgroup is used in the present invention, solid-liquid separation afterthe ink is impacted on paper is smoothly advanced by a synergisticeffect with the organic carboxylic acid salt and water-soluble substanceused in combination, and so excellent results can be achieved in colordeveloping.

As the self-dispersion pigments, pigments having another functionalgroup, for example, a surface modification group composed only of asulfonic acid (or a salt thereof) or a carboxylic acid (or a saltthereof) have been known. When these conventional self-dispersionpigments are compared with the self-dispersion pigment used in thepresent invention, the self-dispersion pigment used in the presentinvention is strong in power for hiding a size scattering in the surfaceof plain paper, and so an outstanding effect is observed in preventionof the so-called blank area phenomenon at a solid print portion.

The self-dispersion pigment used in the present invention has bothlactone group and carboxyl group on the surface of the pigment directlyor through another atomic group. As raw materials before treatment, maybe used pigments of various types. Among others, carbon black ispreferably used. Examples of carbon black include carbon black pigmentssuch as furnace black, lamp black, acetylene black and channel black.

The carbon black pigment preferably has the following characteristics:the primary particle size is from 15 nm or more to 40 nm or less; thespecific surface area is from 50 m²/g or more to 400 m²/g or less asdetermined according to the BET method; the DBP oil absorption is from40 ml/100 g or more to 200 ml/100 g or less; and the volatile mattercontent is 0.5% by weight or more to 10% by weight or less.

As pigments used in color inks, organic pigments are preferably used. Asspecific examples thereof, may be mentioned the following pigments:

Insoluble azo pigments such as Toluidine Red, Toluidine Maroon, HansaYellow, Benzidine Yellow and Pyrazolone Red; water-soluble azo pigmentssuch as Lithol Red, Helio Bordeaux, Pigment Scarlet and Permanent Red2B; derivatives from vat dyes, such as alizarin, indanthron andThioindigo Maroon; phthalocyanine pigments such as Phthalocyanine Blueand Phthalocyanine Green; quinacridone pigments such as Quinacridone Redand Quinacridone Magenta; perylene pigments such as Perylene Red andPerylene Scarlet; isoindolinone pigments such as Isoindolinone Yellowand Isoindolinone Orange; imidazolone pigments such as BenzimidazoloneYellow, Benzimidazolone Orange and Benzimidazolone Red; pyranthronepigments such as Pyranthrone Red and Pyranthrone Orange; thioindigopigments; condensed azo pigments; diketopyrrolopyrrole pigments; andother pigments such as Flavanthrone Yellow, Acylamide Yellow,Quinophthalone Yellow, Nickel Azo Yellow, Copper Azomethine Yellow,Perinone Orange, Anthrone Orange, Dianthraquinonyl Red and DioxazineViolet.

When organic pigments are numerated by COLOR INDEX (C.I.) numbers, thefollowing pigment may be exemplified. C.I. Pigment Yellow: 12, 13, 14,17, 20, 24, 55, 74, 83, 86, 93, 97, 98, 109, 110, 117, 120, 125, 128,137, 138, 139, 147, 148, 150, 151, 153, 154, 155, 166, 168, 180 and 185;C.I. Pigment Orange: 16, 36, 43, 51, 55, 59, 61 and 71; C.I. PigmentRed: 9, 48, 49, 52, 53, 57, 97, 122, 123, 149, 168, 175, 176, 177, 180,192, 202, 209, 215, 216, 217, 220, 223, 224, 226, 227, 228, 238, 240,254, 255 and 272; C.I. Pigment Violet: 19, 23, 29, 30, 37, 40 and 50;C.I. Pigment Blue: 15, 15:1, 15:3, 15:4, 15:6, 22, 60 and 64; C.I.Pigment Green: 7 and 36; and C.I. Pigment Brown 23, 25 and 26. Amongthese pigments, C.I. Pigment Yellow: 13, 17, 55, 74, 93, 97, 98, 110,128, 139, 147, 150, 151, 154, 155, 180 and 185 as yellow pigments, C.I.Pigment Red: 122, 202 and 209, and C.I. Pigment Violet 19 as magentapigments, and C.I. Pigment Blue: 15:3 and 15:4 as cyan pigments are morepreferable. Quite naturally, other pigments than the above-mentionedpigments may also be used.

Specific examples of production processes for directly introducing thelactone group and carboxyl group into the surface of the pigment includea process, in which carbon black is subjected to an oxidizing treatmentwith sodium hypochlorite as described in Japanese Patent ApplicationLaid-Open No. 2004-346090. According to this process, a carboxylic groupand lactone group, which are hydrophilic group, can be introduced intothe surface of carbon black. In the present invention, such a pigmentcan be particularly preferably used.

The average particle size of the self-dispersion pigment used in thepresent invention is preferably 60 nm or more, more preferably 70 nm ormore, still more preferably 75 nm or more. Also, the average particlesize is preferably 145 nm or less, more preferably 140 nm or less, stillmore preferably 130 nm or less. As a method for measuring the averageparticle size, the average particle size can be measured by means ofFPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.) or Nanotrac UPA150EX (manufactured by NIKKISO) utilizing scattering of laser beam. Inthe case of Nanotrac UPA, the particle size is measured as a 50%cumulative value).

Two or more pigments may be used in combination in the same ink asneeded.

The amount of the above-described self-dispersion pigment added into anink is preferably 0.5% by mass or more, more preferably 1% by mass ormore, still more preferably 2% by mass or more based on the total massof the ink. Further, the amount is preferably 15% by mass or less, morepreferably 10% by mass or less, still more preferably 8% by mass orless.

(Organic Carboxylic Acid Salt)

The ink used in the present invention contains an organic carboxylicacid salt. The self-dispersion pigment having both lactone group andcarboxyl group is easy to cause solid-liquid separation after impactedon paper compared with the conventional self-dispersion pigment. thesolid-liquid separation is particularly advanced by using the organiccarboxylic acid salt in combination. As a result, the pigment is fixedto the surface layer of the paper, which can contribute to prevention ofbleeding and high color development. In addition, power for hiding asize scattering in the surface of plain paper strengthens, and so anoutstanding effect is observed in prevention of the so-called blank areaphenomenon at a solid print portion.

No particular limitation is imposed on the organic carboxylic acid saltso far as it is a salt of a carboxylic acid with 1 to 3 carboxyl groupsbonded to a skeleton having carbon atom(s). Specific examples of thecarboxylic acid include citric acid, succinic acid, benzoic acid, aceticacid, phthalic acid, oxalic acid, tartaric acid, gluconic acid,tartronic acid, maleic acid, malonic acid, adipic acid and derivativesthereof. Among these acids, organic carboxylic acid skeletons exhibitinga pH value of from 3 or more to 5 or less as an acid form are morepreferable. As the counter ion for forming the salt, an alkali metal,ammonium or organic ammonium may be used like the case of the counterion in the self-dispersion pigment. The same counter ion as the counterion in the self-dispersion pigment added to the same ink is preferablyused as the counter ion of the organic carboxylic acid.

Specific examples of the alkali metal as the counter ion include Li, Na,K, Rb and Cs. Specific examples of the organic ammonium includemethylammonium, dimethylammonium, trimethylammonium, ethylammonium,diethylammonium, triethylammonium, monohydroxy-methyl (ethyl)amine,dihydroxymethyl(ethyl)amine and trihydroxymethyl(ethyl)amine. Among thecounter ions, ammonium is particularly preferable.

The amount of the organic carboxylic acid salt added into the ink ispreferably 0.05% by mass or more, more preferably 0.1% by mass or more,still more preferably 0.2% by mass or more. Also, the amount ispreferably 3% by mass or less, more preferably 2% by mass or less, stillmore preferably 1% by mass or less.

(Aqueous Medium)

The ink according to the present invention contains water as anessential component, and the content of water in the ink is preferably30% by mass or more based on the total mass of the ink. Further, thecontent is preferably 95% by mass or less. In addition to water, awater-soluble substance that is an essential component is used incombination to provide an aqueous medium. The water-soluble substance ismiscible with water without undergoing phase separation from water inthe form of a 20% by mass liquid mixed with water and is high inhydrophilicity. Any water-soluble substance easy to evaporate is notpreferable from the viewpoints of solid-liquid separation and theprevention of clogging, and a ware-soluble compound having a vaporpressure of 0.04 mmHg or less at 20° C. is preferable.

The ink according to the present invention contains, as an essentialcomponent, a water-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more as defined by thefollowing equation (A).Coefficient of hydrophilicity-hydrophobicity=[(Water activity value of a20% aqueous solution)−(Molar fraction of water in the 20% aqueoussolution)]/[1−(Molar fraction of water in the 20% aqueoussolution)]  Equation (A)

The water activity value in the equation is represented by theexpression:Water activity value=(Water vapor pressure of the aqueoussolution)/(Water vapor pressure of pure water).

Various methods are available as methods for measuring the wateractivity value. Although the method is not limited to any method, achilled mirror dew point measuring method among others is suitable foruse in measurement of materials used in the present invention. Valuesgiven in the present description are obtained by subjecting a 20%aqueous solution of each water-soluble substance to the measurement at25° C. by means of Aqua Lab CX-3TE (manufactured by DECAGON Co.)according to this measuring method.

According to the Raoult's Law, a rate of vapor pressure depression of adilute solution is equal to a molar fraction of a solute and has noconnection with the kinds of the solvent and the solute, so that themolar fraction of water in an aqueous solution is equal to the wateractivity value. However, when water activity values of aqueous solutionsof various water-soluble substances are measured, the water activityvalues do not often consist with the molar fraction of water.

When the water activity value of an aqueous solution is lower than themolar fraction of water, the water vapor pressure of the aqueoussolution comes to be smaller than a theoretical calculated value, andevaporation of water is inhibited by the presence of a solute. From thisfact, it is found that the solute is a substance great in hydrationforce. When the water activity value of an aqueous solution is higherthan the molar fraction of water to the contrary, the solute isconsidered to be a substance small in hydration force.

The present inventors have paid attention to the fact that the degree ofhydrophilicity or hydrophobicity of a water-soluble substance containedin an ink greatly affects the propelling of solid-liquid separationbetween a self-dispersion pigment and an aqueous medium and various inkperformances. From this fact, the coefficient ofhydrophilicity-hydrophobicity represented by the equation (A) has beendefined. The water activity value is measured on aqueous solutions ofvarious water-soluble substances at a fixed concentration of 20% bymass. The degree of hydrophilicity or hydrophobicity between varioussolutes can be relatively compared by being converted to the equation(A) even when the molecular weights of the solutes and the molarfractions of water are different. Since the water activity value of anaqueous solution does not exceed one (1), the maximum value of thecoefficient of hydrophilicity-hydrophobicity is one (1).

The coefficients of hydrophilicity-hydrophobicity of water-solublesubstances used in ink-jet inks, which are obtained according to theequation (A), are shown in Table 1. However, the water-solublesubstances according to the present invention are not limited only tothese compounds.

TABLE 1 Coefficient of Substance name hydrophilicity-hydrophobicity1,2-Hexanediol 0.97 1,2-Pentanediol 0.93 3-Methyl-1,3-butanediol 0.901,2-Butanediol 0.90 2,4-Pentanediol 0.88 1,6-Hexanediol 0.761,7-Heptanediol 0.73 3-Methyl-1,5- 0.54 pentanediol 1,5-Pentanediol 0.41Trimethylolpropane 0.31 Ethyleneurea 0.30 1,2,6-Hexanetriol 0.281,2,3-Butanetriol 0.22 Sorbitol 0.21 Urea 0.20 Diethylene glycol 0.151,2,4-Butanetriol 0.15 Glycerol 0.11 Diglycerol 0.08 Triethylene glycol0.07 Polyethylene glycol 200 −0.09 Polyethylene glycol 600 −0.43

Water-soluble substances having the intended coefficient ofhydrophilicity-hydrophobicity can be selected for use from among variouskinds of water-soluble substances having suitability for ink jetrecording inks.

The present inventors have carried out an investigation as to therelationship between water-soluble substances different in coefficientof hydrophilicity-hydrophobicity and the performance of various inkswhen the water-soluble substances are contained in the inks. As aresult, the following findings have been obtained.

Printing characteristics of small characters, such as bleeding betweentwo (2) colors and dot gain were extremely improved when a water-solublesubstance high in hydrophobic tendency, which has a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more, is used. Among others,compounds having a glycol structure of a hydrocarbon having 4 to 7carbon atoms were particularly preferable. It is considered that thesewater-soluble substances are relatively small in affinity for water,self-dispersion pigment and cellulose fiber after an ink is impacted onpaper, and so the compounds have the role of strongly propellingsolid-liquid separation between the self-dispersion pigment and theaqueous medium. Among these compounds, 1,2-hexanediol and 1,6-hexanediolare particularly preferable.

On the other hand, it is important from the viewpoint of prevention ofclogging in an orifice that solidification of an ink due to evaporationof water is hard to occur. The present inventors have found that theclogging with the ink in the orifice is prevented by using awater-soluble substance having hydration force of a certain extent ormore in combination. When a water-soluble substance having a coefficientof hydrophilicity-hydrophobicity of 0.25 or less is used in combinationwith the water-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more, these compounds cansynergistically act to inhibit bleeding and dot gain in the resultingimage and further prevent clogging with the ink in the orifice. Amongothers, glycerol, diglycerol and polyethylene glycol having a numberaverage molecular weight of 200 or more are preferable.

The total content of the water-soluble substance (A) having acoefficient of hydrophilicity-hydrophobicity of 0.37 or more and thewater-soluble substance (B) having a coefficient ofhydrophilicity-hydrophobicity of 0.25 or less in the ink is preferably5% by mass or more, more preferably 6% by mass or more, still morepreferably 7% by mass or more. Also, the total content is preferably 40%by mass or less, more preferably 35% by mass or less, still morepreferably 30% by mass or less. A proportion “(A)/(B)” of the content ofthe water-soluble substance (A) to the content of the water-solublesubstance (B) in the ink is preferably 1/20 or more, more preferably1/10 or more, still more preferably 1/5 or more. Also, the proportion ispreferably 20/1 or less, more preferably 10/1 or less, still morepreferably 5/1 or less.

The ink used in the present invention preferably contains awater-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37 inaddition to these water-soluble substances. By such composition, thewater-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37, thewater-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more and the water-solublesubstance having a coefficient of hydrophilicity-hydrophobicity of 0.25or less synergistically act to well inhibit bleeding and dot gain. Inaddition, the clogging with the ink can be better prevented. The contentof the water-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37 in theink is preferably 1% by mass or more, more preferably 2% by mass ormore, still more preferably 3% by mass or more. Furthermore, the contentis preferably 30% by mass or less, more preferably 25% by mass or less,still more preferably 20% by mass or less. As the water-solublesubstance having a coefficient of hydrophilicity-hydrophobicity of 0.26or more, but less than 0.37, trimethylolpropane is particularlypreferable.

(Surfactant)

The ink used in the present invention preferably contains a surfactantfor achieving ejection stability with good balance. In particular, theink preferably contains a nonionic surfactant. Among nonionicsurfactants, polyoxyethylene alkyl ethers and ethylene oxide adducts ofacetylene glycol are particularly preferable. The HLB(hydrophile-lipophile balance) values of these nonionic surfactants are10 or more. The content of the surfactant used in the ink in combinationis preferably 0.1% by mass or more, more preferably 0.2% by mass ormore, still more preferably 0.3% by mass or more. Also, the content ispreferably 5% by mass or less, more preferably 4% by mass or less, stillmore preferably 3% by mass or less.

(Other Additives)

Besides the above components, for example, a viscosity modifier, anantifoaming agent, a preservative, a mildew-proofing agent, anantioxidant and a penetrant may be added as additives to the inkaccording to the present invention, as needed, to provide the ink as anink having desired physical property values.

(Surface Tension)

The surface tension of the ink used in the present invention is 34 mN/mor less. The surface tension of the ink is preferably 32 mN/m or less,more preferably 30 mN/m or less.

Since glossy paper and mat paper that are exclusive paper for ink jethave a porous ink-receptive layer formed on the surface of paper unlikeplain paper, such paper is scarcely affected by the surface tension ofan ink to rapidly advance permeation of the ink.

However, a sizing agent having a water-repellent effect is internallyand/or externally added to plain paper, so that the permeation of an inkis often inhibited. In other words, the plain paper is lower in criticalsurface tension, which is an index to whether the surface can be rapidlywetted with the ink or not, than the exclusive paper for ink jet.

When the surface tension of the ink is higher than 34 mN/m, the surfacetension comes to be higher than the critical surface tension of theplain paper, so that the plain paper is not immediately wetted even whenthe ink impacts the paper, and permeation of the ink is not rapidlystarted. When the surface tension of the ink is high, such an ink ishard to be fixed at high speed even when wettability with paper issomewhat improved to lower a contact angle between the ink and thepaper. Further, such an ink has a tendency to deteriorate the fixabilitythereof. When the surface tension of the ink is 34 mN/m or less, poreabsorption is mainly caused. When the surface tension of the ink ishigher than 34 mN/m, fiber absorption is mainly caused. With respect tothe absorption speed of an ink into paper by absorption of these twotypes, the pore absorption is overwhelmingly faster. Thus, an ink thatmainly causes pore absorption is provided in the present invention,thereby realizing high-speed fixing.

The ink that mainly causes pore absorption is also advantageous in thatbleeding is inhibited when recording is conducted by causing two inks ofdifferent colors to adjoin because the two inks are inhibited fromremaining at the same time on the surface of paper. This ink is alsoadvantageous in that a high image density is achieved.

On the other hand, the surface tension of the ink used in the presentinvention is preferably 20 mN/m or more from another viewpoint ofoperability of the ink. When the surface tension is 20 mN/m or more, ameniscus can be retained in an orifice, so that “ink falling” that theink comes out of an ejection opening and escapes from an orifice can beinhibited.

<Recording Method>

In the recording method of the present invention, the amount of an inkapplied at one time is controlled to a fixed amount of from 0.5 pl ormore to 6.0 pl or less. The amount is preferably 1.0 pl or more, morepreferably 1.5 pl or more. Further, the amount is preferably 5.0 pl orless, more preferably 4.5 pl or less. Any amount less than 0.5 pl is notpreferable because the resulting image may become poor in fixability andwater fastness in some cases. If the amount exceeds 6.0 pl, charactersprinted may be defaced by dot gain in some cases when small charactersof the order of from 2 point (1 point≈0.35 mm) to 5 point are printed.

Since the volume of the ink ejected greatly affects the strike throughof the ink, the volume is important even in respect of the applicationto double side printing. Generally, pores having a size of from 0.1 μmto 100 μm centering around from 0.5 μm to 5.0 μm are distributed inplain paper. Incidentally, the plain paper referred to in connectionwith the present invention means copying paper used in a large amount inprinters and copying machines, such as commercially available wood freepaper, medium grade paper and PPC paper, or bond paper. The permeationphenomenon of an aqueous ink into the plain paper is largely dividedinto fiber absorption that the ink is directly absorbed and permeatedinto the cellulose fiber itself of the plain paper and pore absorptionthat the ink is absorbed and permeated into pores formed betweencellulose fibers. The inks used in the present invention are inks to bemainly permeated by the pore absorption. Therefore, when the ink used inthe present invention is applied to the plain paper and a part of theink comes into contact with largish pores of about 10 μm or more, whichare present on the surface of the plain aper, the ink is concentrated onthe largish pores according to the Lucas-Washburn equation and absorbedand permeated therein. As a result, in this portion, the ink comes toparticularly deeply be permeated, which is extremely disadvantageous inachieving high color development on the plain paper. On the other hand,as an ink becomes smallerin the size of ink droplet, the probability ofan ink droplet coming into contact with the largish pore becomes lower,so that the ink is hard to be concentrated on and absorbed in thelargish pore. Further, even if the ink droplet comes into contact withthe largish pore, the amount of the ink deeply permeated may be small sofar as the ink droplet is small. As a result, an image with high colordevelopment is obtained on the plain paper.

The upper critical value of 6.0 pl of the ink is a value empiricallyobtained by the present inventors. When 6.0 pl of the ink is assumed tobe a sphere, the diameter thereof is about 23 μm when impacted on theplain paper. Taking the distribution condition of the largish pores ofabout 10 μm or more in the plain paper into consideration, it isconsidered that when the diameter of the ink is this diameter or less,the probability of contact between the largish pores and the ink uponimpact becomes low, and so a preferable state without causing deeppermeation of the ink is created.

The fixed amount of the ink, referred to in connection with the presentinvention means an ink ejected in a state that the structures of nozzlesmaking up a recording head are not made different from among thenozzles, and the setting of changing drive energy applied is not made.In other words, in such a state, the amount of the ink applied is fixedeven if ejection is somewhat varied because of an error in production ofapparatus. With the amount of the ink applied made fixed, the permeationdepth of the ink is stabilized, the image density of a recorded imagebecomes high, and image uniformity is improved. On the contrary, theamount of the ink is not fixed according to a system based on thepremise that the amount of the ink applied is varied, so that variationin permeation depth of the ink becomes great because inks different involume mixedly exist. In a high duty portion of a recorded image inparticular, the image uniformity is deteriorated because a portion lowin image density exists in the recorded image due to the variation inthe permeation depth.

As a system suitable for applying an ink in a fixed amount, a thermalink jet system that the ink is applied by the action of thermal energyis preferred from the viewpoint of ejection mechanism. Morespecifically, according to the thermal ink jet system, the variation inpermeation depth of the ink is inhibited, and the resulting recordedimage is high in image density and good in uniformity. In addition, thethermal ink jet system is suitable for achieving a recording head of amulti-nozzle and high-density type and also preferable for high-speedrecording.

The problems of the present invention are then required in the casewhere an image having a portion with a duty of 80% or more is formed ina basic matrix for forming an image. A portion for calculating the dutyis 50 μm×50 μm at the minimum. The image having a portion with a duty of80% or more is an image having a portion formed with an ink applied to80% or more of lattices among lattices in the matrix of the portion forcalculating the duty. The size of lattices is determined by theresolution of the basic matrix. For example, when the resolution of thebasic matrix is 1,200 dpi×1,200 dpi, the size of a lattice is 1/1,200inch× 1/1,200 inch.

The image having the portion with a duty of 80% or more in the basicmatrix is an image having a portion where one color ink is applied in aduty of 80% or more in the basic matrix. More specifically, in the casewhere four (4) color inks of black, cyan, magenta and yellow are used,the image is an image having a portion where at least one color inkthereof are applied in a duty of 80% or more in the basic matrix. On theother hand, an image having no portion with a duty of 80% or more in thebasic matrix has a relatively small overlapping between inks impactedand may not cause problems of character defacement and bleeding in manycases even when the printing process is not devised.

The basic matrix of the present invention can be freely set according tothe recording apparatus. The resolution of the basic matrix ispreferably 600 dpi or more, more preferably 1,200 dpi or more. Also, theresolution is preferably 4,800 dpi or less. The resolution may be thesame or different in length and width so far as it falls within thisrange.

The problems of the present invention are also required in the casewhere an image having a portion wherein the total amount of an inkapplied is 5.0 μl/cm² or less is formed in a basic matrix for forming animage. A portion for calculating the total amount of the ink applied isthe same as the portion for calculating the duty. If an image having aportion that the total amount of the ink applied exceeds 5.0 μl/cm² isformed, in some cases, a clear image may not be obtained, or strikethrough may occur, which is disadvantageous for double side printing.

In the present invention, the application of the ink is divided intoplural times when an image having a portion where the duty is 80% ormore and the total amount of an ink applied is 5.0 μl/cm² or less isformed in the basic matrix for forming the image. The amount of the inkapplied at each of the times divided is 0.7 μl/cm² or less, preferably0.6 μl/cm² or less, more preferably 0.5 μl/cm² or less. If the amount ofthe ink applied to the image at each time exceeds 0.7 μl/cm², strikethrough, character defacement and/or bleeding may occur in some cases.

The reason why the application of the ink divided into the plural timesin the formation of the image is an essential requirement in the presentinvention is based on the fact that there is a particular difference inperformance between the case where the application is divided and thecase where the application is not divided. The number of times ofdivision of the ink application is at least 2 times or more. When thenumber of times of division is 3 times or more, the resulting recordedimage becomes higher in density and good color developing. The number oftimes is preferably 8 times or less, more preferably 4 times or less. Ifthe number of times of division exceeds 8 times, there is a tendencythat the covering rate of the ink on the surface of plain paper islowered to deteriorate color developing though such application iseffective for inhibition of bleeding and good printing of smallcharacters.

<Ink Jet Recording Apparatus>

The ink jet recording apparatus according to the present invention willhereinafter be described. An apparatus suitable for use in the presentinvention is an apparatus equipped with a recording head for applying anink in a fixed amount of from 0.5 pl or more to 6 pl or less. Therecording head of the ink jet recording apparatus according to thepresent invention is preferably a recording head that thermal energy iscaused to act on an ink to apply the ink. Such a recording head issuitable for forming nozzles at a high density compared with a recordinghead that an ink is ejected by using a piezoelectric element. Inaddition, the recording head is excellent in applying the ink in thefixed amount and thus excellent in that variation in permeation depth ofthe ink is reduced, and the uniformity of the resulting recorded imageis made good.

With respect to the typical construction and principle of the recordinghead that thermal energy is caused to act on an ink to apply the ink,those using the basic principle disclosed in, for example, U.S. Pat.Nos. 4,723,129 and 4,740,796 are preferable. This system may be appliedto any of the so-called On-Demand type and continuous type. Inparticular, the On-Demand type is more advantageous because at least onedriving signal, which corresponds to recording information and gives arapid temperature rise exceeding nuclear boiling, is applied to anelectrothermal converter arranged corresponding to a sheet or a liquidpath, in which an ink is retained, thereby causing the electrothermalconverter to generate thermal energy to cause film boiling on theheat-acting surface of a recording head, so that a bubble can be formedin the ink in response to the driving signal in relation of one to one.The ink is ejected through an ejection opening by the growth-contractionof this bubble to form at least one droplet. When the driving signal isapplied in the form of a pulse, the growth-contraction of the bubbleproperly occurs in a moment, so that the amount of the ink ejected isfixed, and the ejection of the ink, which is also excellent inresponsiveness, can be achieved. It is therefore preferable to use suchpulsed signals.

FIG. 1 is a front elevation schematically illustrating an ink jetrecording apparatus according to an embodiment of the present invention.A plurality of recording heads 211 to 214 of an ink jet system ismounted on a carriage 20. A plurality of ink ejection openings forejecting an ink is arranged in each of the recording heads 211 to 214.The recording heads 211, 212, 213 and 214 are recording heads forejecting cyan (C), magenta (M), yellow (Y) and black (K) inks,respectively.

Ink cartridges 221 to 224 are respectively constructed by the recordingheads 211 to 214 and ink tanks for feeding inks to these recordingheads.

A concentration sensor 40 is provided. The concentration sensor 40 is areflection type concentration sensor and is so constructed that thedensity of a test pattern recorded on a recording medium can be detectedin a state provided on a side surface of the carriage 20.

Control signals to the recording heads 211 to 214 are transferredthrough a flexible cable 23.

A recording medium 24, to the surface of which cellulose fiber isexposed, such as plain paper is held by discharge rollers 25 viaconveyance rollers (not illustrated) and conveyed in a direction(secondary scanning direction) of an arrow by driving a conveyance motor26.

The carriage 20 is guided and supported by a guide shaft 27 and a linearencoder 28. The carriage 20 is reciprocatingly moved in a main scanningdirection along the guide shaft 27 through a drive belt 29 by driving acarriage motor 30.

A heating element (electricity-thermal energy converter) for generatingthermal energy for ink ejection is provided in the interior (liquidpath) of each of the ink ejection openings of the recording heads 211 to214. The heating element is driven based on a recording signal accordingto reading timing of the linear encoder 28 to eject and apply inkdroplets to the recording medium, thereby forming an image.

A recovery unit 32 having cap parts 311 to 314 is provided at a homeposition of the carriage 20 arranged outside a recording region. Whenrecording is not conducted, the carriage 20 is moved to the homeposition, and the ink ejection opening faces of the recording heads 211to 214 are closed by their corresponding caps 311 to 314, wherebysticking of the inks caused by evaporation of ink solvents or cloggingby adhesion of foreign matter such as dust can be prevented. The cappingfunction of the cap parts is utilized for solving ejection failure orclogging of ejection openings low in recording frequency. Specifically,the capping parts are utilized for blank ejection for preventingejection failure, in which the inks are ejected to the cap parts locatedin a state separated from the ink ejection openings. Further, the capparts are utilized for sucking the inks from the ink ejection openingsin a capped state by a pump (not illustrated) to recover ejection ofejection openings undergone ejection failure.

An ink receiving part 33 plays the role of receiving ink dropletspreliminarily ejected when the recording heads 211 to 214 pass throughover it just before recording operation. A blade or wiping member (notillustrated) is arranged at a position adjoining the cap parts, wherebythe ink ejection opening-forming faces of the recording heads 211 to 214can be cleaned.

As described above, it is preferable to add the recovery unit forrecording heads and preliminary units to the construction of therecording apparatus because the recording operation can be morestabilized. Specific examples of these units include capping units,cleaning units and pressurizing or sucking units for recording heads,and preliminary heating units by electrothermal converters, otherheating elements than these converters or combinations thereof. It isalso effective for stably conducting recording to provide a preliminaryejection mode that ejection separate from recording is conducted.

In addition, a cartridge type recording head, in which an ink tank isprovided integrally with the recording head itself described in theabove-described embodiment may also be used. Further, a replaceable chiptype recording head, in which electrical connection to an apparatus bodyand the feed of an ink from the apparatus body become feasible byinstalling it in the apparatus body, may also be used.

FIG. 2 illustrates the construction of the recording heads 211 to 214.In the drawing, the recording scan directions of the recording heads 211to 214 are directions indicated by arrows. A plurality of nozzles, i.e.,ejection openings, arranged in a direction substantially perpendicularto the recording scan direction is provided in each of the recordingheads 211 to 214. Each recording head ejects ink droplets atpredetermined timing from the respective ejection openings while beingmoved and scanned in a recording scan direction in the drawing, wherebyan image is formed on a recording medium with a recording resolutionaccording to the arrangement density of the nozzles. At this time, therecording head may conduct recording operation in any direction of therecording scan directions. The recording operation may be conducted inany direction of forward and return directions.

The above-described embodiment is a recording apparatus of the serialtype that a recording head is scanned to conduct recording. However, arecording apparatus of the full-line type that a recording head having alength corresponding to the width of a recording medium is used may alsobe used. As a recording head of the full-line type, there isconstruction that such recording heads of the serial type as disclosedin FIG. 2 are arranged in a zigzag state or in parallel to form acontinuous recording head so as to give the intended length.Alternatively, construction that one recording head integrally formed soas to have a continuous nozzle row is used may also be adopted.

The above-described recording apparatus of the serial type or line typeis an embodiment that a head of the construction of 4 ejection openingrows (or nozzle rows) independently or integrally formed and using 4color inks (Y, M, C and K) is installed. It is also preferable that thenumber of ejection opening rows is increased to about 5 to 12, and aboutat least one ink of the 4 color inks, inks of the same color areduplicatively charged in plural ejection opening rows (or nozzle rows).Examples thereof include construction of 8 ejection opening rows (ornozzle rows) and construction of 12 ejection opening rows (or nozzlerows) that 2 or 3 heads of the construction illustrated in FIG. 2 arecontinuously connected.

According to the ink jet recording apparatus of the present invention,the application of the ink is divided into plural times when an imagehaving a portion where the duty is 80% or more and the total amount ofan ink applied is 5.0 μl/cm² or less is formed in a basic matrix forforming the image. The amount of the ink applied at each of the timesdivided is controlled to 0.7 μl/cm² or less. The ink jet recordingapparatus of the present invention has a control mechanism forconducting such divided application of ink. The operation of the ink jetrecording head and the timing of conveyance operation of plain paper arecontrolled by this control mechanism to conduct such divided applicationof ink.

The number of times of division in the application of the ink may be setaccording to desired recording conditions. An example where theapplication of ink is divided into 2 times is illustrated in FIG. 3.This example is an example where the resolution of a basic matrix is1,200 dpi (width)×1,200 dpi (length), and an image having a portionwhere the duty is 100% is formed. In FIG. 3, the impact positions of theink applied at the first time and the impact positions of the inkapplied at the second time are illustrated as the first ink and thesecond ink, respectively. The first ink and second ink are respectivelyapplied in a fixed amount.

EXAMPLES

The present invention will hereinafter be described specifically by thefollowing Examples and Comparative Examples. Incidentally, alldesignations of “part” or “parts” and “%” in the following examples meanpart or parts by mass and % by mass unless expressly noted.

First of all, preparation processes of pigment dispersions contained ininks used in Examples and Comparative Examples are described.

(Preparation of Pigment Dispersion)

<Preparation of Self-Dispersion Pigment Dispersion A>

Carbon black (500 g) having a specific surface area of 220 m²/g and aDBP oil absorption of 160 ml/100 g was added to ion-exchanged water(3,750 g), and the resultant mixture was heated to 50° C. while stirringit. Thereafter, an aqueous solution of sodium hypochlorite (availablechlorine concentration: 12%; 4,500 g) was added dropwise over 3 hours at50° C. while being pulverized by a bead mill using zirconia beads havinga diameter of 0.5 mm. The pulverization was then conducted for 30minutes to obtain a reaction liquid containing self-dispersion carbonblack. After the reaction liquid was fractionated, neutralization wasconducted with aqueous ammonia, and desalting was conducted by anultrafilter until conductivity became 1.5 mS/cm. After the thus-treatedliquid was adjusted so as to give a self-dispersion carbon blackconcentration of 10%, the liquid was filtered by using a prefilter and afilter having a pore size of 1 μm in combination to obtain aself-dispersion pigment dispersion A.

<Preparation of Self-dispersion Pigment Dispersion B>

A self-dispersion pigment dispersion B was obtained in the same manneras in the preparation of the self-dispersion pigment dispersion A exceptthat carbon black having a specific surface area of 320 m²/g and a DBPoil absorption of 110 ml/100 g was used.

<Preparation of Ink Y for Evaluation and Investigation>

After the following components (100 parts in total) were mixed for 3hours, the resultant mixture was adjusted to pH 8.5 with sodiumhydroxide and filtered through a filter having a pore size of 0.45 μm.

-   -   C.I. Direct Yellow 86: 2 parts    -   Glycerol: 10 parts    -   Diethylene glycol: 10 parts    -   Isopropyl alcohol: 2 parts    -   Ethylene oxide adduct of acetylene glycol (trade name: Olfin        E1010, product of Nisshin Chemical Industry Co., Ltd.): 1 part    -   Water: balance.

Preparation Examples of Inks for Examples of the present invention andComparative Examples are then described.

Example 1 Preparation of Ink 1

After the following components (100 parts in total) were mixed for 2hours, the resultant mixture was adjusted to pH 8.5 with aqueous ammoniaand filtered through a filter having a pore size of 2.5 μm to obtain anink 1 of Example of the present invention. The surface tension of theink was 29 mN/m, and the average particle size of the self-dispersionpigment was 140 nm.

-   -   Self-dispersion pigment dispersion A: 40 parts    -   Glycerol (coefficient of hydrophilicity-hydrophobicity: 0.11):        16 parts    -   1,6-Hexanediol (coefficient of hydrophilicity-hydrophobicity:        0.76): 4 parts    -   Ammonium benzoate: 0.3 parts    -   Isopropyl alcohol: 1 part    -   Ethylene oxide adduct of acetylene glycol (trade name: Olfin        E1010, product of Nisshin Chemical Industry Co., Ltd., HLB        value: 10 or more): 1 part    -   Triethylene glycol monobutyl ether: 0.5 parts    -   Water: balance.

Example 2 Preparation of Ink 2

After the following components (100 parts in total) were mixed for 2hours, the resultant mixture was adjusted to pH 8.5 with aqueous ammoniaand filtered through a filter having a pore size of 2.5 μm to obtain anink 2 of Example of the present invention. The surface tension of theink was 29 mN/m, and the average particle size of the self-dispersionpigment was 140 nm.

-   -   Self-dispersion pigment dispersion A: 40 parts    -   Glycerol (coefficient of hydrophilicity-hydrophobicity: 0.11): 8        parts    -   Trimethylolpropane (coefficient of        hydrophilicity-hydrophobicity: 0.31): 8 parts    -   1,2-Hexanediol (coefficient of hydrophilicity-hydrophobicity:        0.97): 4 parts    -   Diammonium phthalate: 0.5 parts    -   Isopropyl alcohol: 1 part    -   Ethylene oxide adduct of acetylene glycol (trade name: Olfin        E1010, product of Nisshin Chemical Industry Co., Ltd., HLB        value: 10 or more): 1 part    -   Triethylene glycol monobutyl ether: 0.5 parts    -   Water: balance.

Example 3 Preparation of Ink 3

After the following components (100 parts in total) were mixed for 2hours, the resultant mixture was adjusted to pH 8.5 with aqueous ammoniaand filtered through a filter having a pore size of 2.5 μm to obtain anink 3 of Example of the present invention. The surface tension of theink was 29 mN/m, and the average particle size of the self-dispersionpigment was 120 nm.

-   -   Self-dispersion pigment dispersion B: 40 parts    -   Trimethylolpropane (coefficient of        hydrophilicity-hydrophobicity: 0.31): 10 parts    -   1,2-Pentanediol (coefficient of hydrophilicity-hydrophobicity:        0.93): 10 parts    -   Isopropyl alcohol: 1 part    -   Ammonium acetate: 0.4 parts    -   Ethylene oxide adduct of acetylene glycol (trade name: Olfin        E1010, product of Nisshin Chemical Industry Co., Ltd., HLB        value: 10 or more): 1 part    -   Triethylene glycol monobutyl ether: 0.5 parts    -   Water: balance.

Example 4 Preparation of Ink 4

An ink 4 of Example of the present invention was obtained in the samemanner as in the preparation of the ink 1 except that theself-dispersion pigment dispersion B was used in place of theself-dispersion pigment dispersion A. The surface tension of the ink was29 mN/m, and the average particle size of the self-dispersion pigmentwas 120 nm.

Example 5 Preparation of Ink 5

An ink 5 of Example of the present invention was obtained in the samemanner as in the preparation of the ink 2 except that theself-dispersion pigment dispersion B was used in place of theself-dispersion pigment dispersion A. The surface tension of the ink was29 mN/m, and the average particle size of the self-dispersion pigmentwas 120 nm.

Comparative Example 1

Preparation of Ink 6

An ink 6 of Comparative Example was obtained in the same manner as inthe preparation of the ink 4 except that the content of the ethyleneoxide adduct of acetylene glycol was changed from 1 part to 0.1 part.The surface tension of the ink was 36 mN/m, and the average particlesize of the self-dispersion pigment was 115 nm.

Comparative Example 2

Preparation of Ink 7

An ink 7 of Comparative Example was obtained in the same manner as inthe preparation of the ink 5 except that diammonium phthalate was notadded. The surface tension of the ink was 29 mN/m, and the averageparticle size of the self-dispersion pigment was 115 nm.

Comparative Example 3

Preparation of Ink 8

An ink 8 of Comparative Example was obtained in the same manner as inthe preparation of the ink 4 except that 1,6-hexanediol was changed totriethylene glycol (coefficient of hydrophilicity-hydrophobicity: 0.07).The surface tension of the ink was 29 mN/m, and the average particlesize of the self-dispersion pigment was 125 nm.

The inks 1 to 5 of Examples and the inks 6 to 8 of Comparative Exampleswere used to evaluate recorded articles under the following conditions,thereby providing examples of image forming methods and recordingapparatus.

(Evaluation Conditions)

Recording medium: Office Planner paper for common use in PPC/BJ (plainpaper; product of Canon Marketing Japan Inc.)

Printer A: F930 (manufactured by Canon Inc.; recording head: 6 ejectionopening rows, including 512 nozzles in each row; amount of the ink: 4.0pl (fixed amount); resolution of the basic matrix: 1,200 dpi(width)×1,200 dpi (length))

Image forming method: Each of the inks 1-8 was charged in a black inkhead part of the printer, and the ink Y for evaluation and investigationwas charged in a yellow ink head part to print a solid print image. Uponthe formation of the solid print image, the application of each ink wasdivided into 4 times, the amount of the ink applied at one time was afixed amount of 0.3 μl/cm², and the total amount of the ink applied was1.2 μl/cm².

Recorded articles in Examples 1 to 5 and Comparative Examples 1 to 3were evaluated as to image density (O.D.), fixability, bleeding, smallcharacter printing and uniformity, and the results are shown in Table 2.Evaluation as to images was made by using a black head and printing asolid print image (3 cm×3 cm) and JIS first level Chinese characters of5 point. Incidentally, evaluating methods and standards of the recordingare as follows.

(Image Density)

O.D. of a solid print image was measured by a densitometer (MacbethRD915; manufactured by Macbeth Co.).

-   A: O.D. was 1.40 or more;-   B: O.D. was 1.35 or more, but less than 1.40;-   C: O.D. was 1.30 or more, but less than 1.35;-   D: O.D. was less than 1.30.    (Fixability)

After 10 seconds from the printing of the solid print image, silbonpaper was put under pressure on the print to visually evaluate thedegree of transfer according to the following evaluation standard.

-   A: No transfer is observed;-   B: Transfer is slightly observed;-   C: Transfer is clearly observed.    (Bleeding)

The same solid print image as the black solid print image was printedadjoining the black solid print image with the ink Y for evaluation andinvestigation from the yellow head under the same conditions as in theblack head to visually observe the condition of the boundary portionthereof.

-   A: Bleeding is not observed;-   B: Bleeding is slightly observed, but it is within an allowable    range;-   C: Bleeding is observed;-   D: Bleeding is markedly observed.    (Small Character Printing)

The sharpness of small characters (Chinese characters) printed wasvisually evaluated according to the following evaluation standard.

-   A: Even complex small characters can also be expressed;-   B: Outlines are slightly disordered in complex small characters, but    it is within an allowable range;-   C: Complex small characters cannot be sufficiently expressed;-   D: Disorder may be observed even in simple small characters in some    cases.    (Uniformity)

The degree of occurrence of blank area in the solid print image wasvisually evaluated according to the following evaluation standard.

-   A: No blank area is observed;-   B: Blank area is slightly observed;-   C: Blank area is clearly observed.

TABLE 2 Small Image character Ink density Fixability Bleeding printingUniformity Ex. 1 1 A A A A A Ex. 2 2 A A A A A Ex. 3 3 B A A A A Ex. 4 4A A A A A Ex. 5 5 A A A A A Comp. 6 C C C A B Ex. 1 Comp. 7 B A C B BEx. 2 Comp. 8 A A C C B Ex. 3

When the results of Examples 1 to 5 are compared with those ofComparative Example 1, it is understood that when the surface tension ofthe ink is controlled to 34 mN/m or less, image density is made high,fixability and uniformity are improved, and bleeding is well inhibited.When the results of Examples 1 to 5 are compared with those ofComparative Example 2, it is understood that the inks according to thepresent invention contain the organic carboxylic acid salt, wherebyimage density is made high, bleeding is well inhibited, small characterprinting is well conducted, and uniformity is improved. When the resultsof Examples 1 to 5 are compared with those of Comparative Example 3, itis understood that the inks according to the present invention containthe water-soluble substance having a coefficient ofhydrophilicity-hydrophobicity of 0.37 or more, whereby bleeding is wellinhibited, small character printing is well conducted, and uniformity isimproved.

The respective inks prepared in Examples 1, 2 and 4 and theircorresponding conditions shown in the following Tables 3 and 4 werecombined to provide examples of image forming methods and recordingapparatus for performing such methods in Examples 6 to 15. Table 3 showsexamples where the amount of the ink applied was divided into equalamounts at respective times. Table 4 shows examples where the amounts ofthe ink applied at the respective times are not equal. The evaluationitems and methods are the same as in Examples 1 to 5. The results areshown in Table 5.

A printer B for evaluation added is F950 (manufactured by Canon Inc.;recording head: 6 ejection opening rows, including 512 nozzles in eachrow; amount of the ink: 2.0 pl (fixed amount); resolution of the basicmatrix: 2,400 dpi (width)×1,200 dpi (length)).

TABLE 3 Divided Amount applied at Total amount Ink Printer times eachtime (μl/cm²) applied (μl/cm²) Ex. 6 2 A 2 0.5 1.0 Ex. 7 2 A 3 0.333 1.0Ex. 8 2 A 5 0.2 1.0 Ex. 9 2 A 8 0.125 1.0 Ex. 10 1 A 5 0.2 1.0 Ex. 11 2A 8 0.375 3.0 Ex. 12 2 A 8 0.4 3.2

TABLE 4 Amount applied at each Divided time (μl/cm²) Total amount InkPrinter times 1st 2nd 3rd 4th applied (μl/cm²) Ex. 13 4 B 4 0.3 0.3 0.30.3 1.2 Ex. 14 4 B 4 0.2 0.1 0.1 0.7 1.2 Ex. 15 4 B 4 0.5 0.1 0.1 0.51.2

TABLE 5 Small Image character density Fixability Bleeding printingUniformity Ex. 6 B A B B A Ex. 7 A A A A A Ex. 8 A A A A A Ex. 9 A A A AA Ex. 10 A A A A A Ex. 11 A A A A A Ex. 12 A A A B A Ex. 13 A A A A AEx. 14 B A A A A Ex. 15 A A A A A

It is understood from the results shown in Table 5 that the ink jetrecording method according to the present invention yields good resultson all the image density, fixability, bleeding, small character printingand uniformity.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application Nos.2007-191038, filed Jul. 23, 2007 and 2008-036152, filed Feb. 18, 2008,which are hereby incorporated by reference in their entirety.

1. An ink jet recording ink comprising: a self-dispersion pigment havinga lactone group and a carboxyl group; an ammonium salt of an organiccarboxylic acid; water; and a water-soluble substance having acoefficient of hydrophilicity-hydrophobicity of 0.37 or more as definedby the following equation (A), and having a surface tension of 34 mN/mor less:Coefficient of hydrophilicity-hydrophobicity =[(Water activity value ofa 20% aqueous solution) −(Molar fraction of water in the 20% aqueoussolution)]/ [1−(Molar fraction of water in the 20% aqueoussolution)].  Equation (A)
 2. The ink jet recording ink according toclaim 1, which further comprises a water-soluble substance having acoefficient of hydrophilicity-hydrophobicity of 0.25 or less as definedby the equation (A).
 3. The ink jet recording ink according to claim 2,which further comprises a water-soluble substance having a coefficientof hydrophilicity-hydrophobicity of 0.26 or more, but less than 0.37 asdefined by the equation (A).
 4. The ink jet recording ink according toclaim 1, which further comprises a water-soluble substance having acoefficient of hydrophilicity-hydrophobicity of 0.26 or more, but lessthan 0.37 as defined by the equation (A).
 5. The ink jet recording inkaccording to claim 1, wherein the average particle size of theself-dispersion pigment is from 60 nm or more to 145 nm or less.
 6. Theink jet recording ink according to claim 1, wherein an amount of theammonium salt of the organic carboxylic acid added into the ink is 0.05%by mass or more and 3% by mass or less.
 7. An ink jet image-formingmethod for forming an image by applying the ink jet recording inkaccording to claim 1 in a fixed amount of from 0.5 pl or more to 6.0 plor less to plain paper, wherein the application of the ink is dividedinto plural times when an image having a portion where the ink isapplied in a duty of 80% or more and in an amount of 5.0 μl/cm² or lessin total is formed in a basic matrix for forming the image, and theamount of the ink applied at each of the times divided is 0.7 μl/cm² orless.
 8. The ink jet image-forming method according to claim 7, whereinthe resolution of the basic matrix is from 600 dpi or more to 4,800 dpior less.
 9. The ink jet image-forming method according to claim 7,wherein the number of times of division is from 2 times or more to 8times or less.
 10. The ink jet image-forming method according to claim7, wherein the application of the ink is conducted by the action ofthermal energy.
 11. An ink jet recording apparatus equipped with (a) anink cartridge storing the ink jet recording ink according to claim 1 and(b) a recording head for forming an image by applying the ink jetrecording ink according to claim 1 in a fixed amount of from 0.5 pl ormore to 6.0 pl or less to plain paper, wherein the apparatus comprises acontrol mechanism, by which the application of the ink is divided intoplural times when an image having a portion where the ink is applied ina duty of 80% or more and in an amount of 5.0 μl/cm² or less in total isformed in a basic matrix for forming the image, and the amount of theink applied at each of the times divided is controlled to 0.7 μl/cm² orless.
 12. The ink jet recording apparatus according to claim 11, whereinthe recording head is a recording head that the application of the inkis conducted by the action of thermal energy.